The present invention relates to a wafer sensing apparatus, and more particularly, to a wafer sensing apparatus for sensing whether or not a wafer is inserted in a wafer cassette, thereby preventing the malfunctioning of a robot arm due to the faulty sensing of the wafer insertion. Such a wafer sensing apparatus would result in an improvement in equipment operating efficiency.
Generally, the transfer process of loading and unloading a wafer from a piece of semiconductor process equipment--used for fabricating a semiconductor device--is performed automatically using a robot arm to prevent the generation of polluting particles. In this transfer process, it is essential to quickly and accurately determine whether or not a wafer is inserted into respective wafer holding slots formed in a wafer cassette.
The wafer insertion sensing operation is performed by a wafer sensing apparatus having a light emitting means for emitting light toward the wafer cassette, with the emitted light being parallel with respect to the slots. A light sensing means opposing the light emitting means senses the light emitted from the light emitting means.
Suppose for example, that through an error, another wafer has previously been inserted into a slot of the wafer cassette into which a wafer completing a predetermined process should be inserted. In this case, the light sensing means cannot sense the light emitted from the light emitting means since the light is blocked by the previously inserted wafer. In such a case the wafer transfer operation would stop with an error message, such as "CASSETTE SLOT NOT EMPTY", being generated.
However, if the wafer transferring operation continued under the above situation despite the wafer error message, due to a problem with the wafer sensing apparatus for example, it could result in two wafers that are trying to be inserted in the same cassette slot, whereby the wafers or a pair of wafer transferring tweezers made of quartz are susceptible to being broken.
On the other hand, an error could also occur even if there were no previously inserted wafer in the wafer cassette before a wafer completing a predetermined process was to be inserted into the corresponding wafer cassette slot. For example, an error in the wafer sensing apparatus could cause the transfer operation to stop automatically, with an error message such as "CASSETTE SLOT NOT EMPTY" as described above being generated, even though no actual wafer was in the slot. This would result in a lowering of the equipment working rate.
FIG. 1 is a perspective view schematically showing a conventional wafer sensing apparatus and the wafer sensing operation thereof.
Referring to FIG. 1, a conventional wafer sensing apparatus is constituted by a light emitting means 3 and a light sensing means 5. The light sensing means 5 includes a main body 6, a plurality of aperture slots 7, and light sensing devices (not shown). A plurality of wafers 1 are inserted into slots formed in a wafer cassette 4. The wafer transferring operation is performed by a robot arm 2 which detachably attaches to the rear side of each wafer 1 using vacuum suction. The light emitting device 3, positioned under the robot arm 2, emits light a toward the inner portion of the wafer cassette 4. The emitted light a either passes through the wafer cassette 4 via an empty slot, or is blocked by the wafer 1. The passed light a also passes through one of the plurality of aperture slots 7 formed on the surface of the main body 6, and is then sensed by the light sensing devices (not shown). The location of the light sensing devices is determined according to the respective aperture slots 7.
Generally, the light sensing device can sense a light in a predetermined wavelength range. For example, an infrared ray sensor senses a light in a wavelength range of about 800 to 1100 nm. However, in the conventional wafer sensing apparatus for sensing whether the wafer is inserted or not, having the structure described above, the light sensing device may erroneously sense whether the wafer is inserted due to interference from interference light b. Such interference light may come from a number of outside sources, such as overhead or space lighting from the fabrication facility where the processing equipment is located, or radiant light from the infrared rays emitted from a highly heated piece of processing equipment. When the wafer sensing apparatus malfunctions due to the faulty sensing of the light sensing device, the equipment operating efficiency is lowered, and breakage of the wafer or wafer handling devices may occur.